Brothers compete. So in 2016, when astronaut Scott Kelly returned to Earth after spending a year in space, it must have really annoyed his identical twin brother — retired astronaut Mark Kelly — that Scott was two inches taller than when he left. However, Scott’s temporary increase in height was not the only thing that changed during his trip.

As part of NASA’s Twins Study, while Scott was in space, Mark went about his daily life on Earth. Over the course of the year-long mission, researc